With the remarkable development of small, thin, and high-functionality portable electronic equipment such as mobile phones and PDAs, there is a growing demand for smaller, thinner, and high-capacity batteries as their power source. Lithium ion rechargeable batteries can be designed small and high-capacity, and in particular, flat prismatic types are suitable in making the equipment thinner; they have therefore been increasingly used as the repeatedly usable rechargeable battery for portable electronic equipment.
Because lithium ion rechargeable batteries have high energy density and contain a flammable organic solvent as electrolyte, it is essential to take account of safety measures. They must have such safety features as to ensure that no damage is caused to the equipment or injury to the user in the event that an abnormality arises for some reason. For example, if the positive and negative terminals of the battery are short-circuited for some reason, a large short-circuit current flows in high energy density batteries, whereupon the inner resistance generates Joule heat and the battery temperature rises. A temperature rise in the battery leads to a rapid increasing of gas pressure inside resulting from reactions between positive electrode active materials and electrolyte, or from evaporation or decomposition of electrolyte, which may cause fire or explosion of the battery. Batteries may fall into a high-temperature state not only because of external short-circuiting but also of overcharge; the same applies if the portable electronic equipment loaded with the battery is placed near a heater or left inside a car parked in a hot weather environment.
A battery abnormality can be induced by any of electrical, mechanical, or thermal factors; thus non-aqueous electrolyte batteries represented by lithium ion rechargeable batteries are provided with safety features for preventing batteries from falling into an abnormal state and for evading a further dangerous state even if an abnormality should arise. Such features are usually incorporated in batteries as their own natures; for example, active materials on the electrodes and electrolyte may be made not to be excessively reactive, or, a polyolefin porous film may be employed for the separator because of its “shutdown function,” in which minute pores are softened and close under an abnormally high temperature. Cylindrical lithium ion rechargeable batteries are usually provided with a protective feature such as a Positive Temperature Coefficient (PTC) element connected in series to the input/output circuit at the sealing end, which limits current flow in the event of external short-circuiting. Batteries that do not include the PTC element inside are normally provided with a PTC element or temperature fuse as outside circuit components. Further, a circuit for protecting the battery from overcharge and over discharge is an absolute requirement. In general, these constituent elements are all packed with the battery inside a pack case to form a battery pack.
However, battery packs using pack cases are not suited to portable electronic equipment that are re-modeled in short cycles, because the manufacturing cost of molding dies used in the resin molding of pack cases tends to be high, and the time required for designing new molding dies is relatively long. Thinner battery packs with resin-molded pack case as an outer case also have limitations in making portable electronic equipment smaller and thinner because of the limitations on the moldable thickness in the resin molding process.
Furthermore, in order to prevent the user from disassembling a battery pack for wrong use or for satisfying curiosity, it must have a design that is hardly disassemblable, or a design that alerts the user that it has been disassembled. Taking account that the battery packs are used for portable electronic equipment, they also need to have a rigid structure that can withstand vibration or shocks in a falling accident, and a moisture resistance, particularly for the electronic circuit parts. In achieving the structure having a disassemblablity, a certain rigidity, and a moisture resistant, the idea has emerged that a battery may be united with a circuit substrate including a battery protective circuit by resin molding.
The applicants of the present invention have disclosed resin-molded battery packs in Japanese Laid-Open Patent Publications Nos. 2002-134077 and 2002-166447, in which a battery and a circuit substrate are connected by a connecting member to form an intermediate product, which is placed inside a die, and resin is filled around the intermediate product such as to expose external terminals formed on the substrate to the outside, to unite the battery with the substrate.
Japanese Laid-Open Patent Publication No. 2000-315483 discloses a structure in which a battery and a circuit substrate are connected by a connecting member and placed inside a die, and the circuit substrate is resin-sealed and fixed on the battery or its pack case (battery lid), or both the circuit substrate and the battery are resin-sealed.
Resin needs to be engaged with the circuit substrate and the rechargeable battery when uniting them by resin molding. While the circuit substrate has various protrusions and recesses because of electronic components and lead plates connected thereto and can retain molded resin well, the rechargeable battery has fewer such protrusions and recesses even at the sealing part; because the rechargeable battery is formed of a metal material with a flat surface that can hardly form a good bond with resin, the filled resin can easily come off after curing, resulting in unstable bond between the circuit substrate and the rechargeable battery. The rechargeable battery thus needs to be provided with some protrusions and recesses to retain the filled resin.
However, none of the above prior art examples shows a structure for retaining filled resin on the rechargeable battery; the resin may peel off when the rechargeable battery is subjected to an impact in a falling accident or the like and the circuit substrate may separate from the rechargeable battery, or, the battery may be disassembled for satisfying curiosity and put to wrong use because of the ease of disassembly.
It is an object of the invention to provide a battery formed with protrusions and/or recesses that function as engaging parts for uniting a circuit substrate with the battery by resin molding, and a manufacturing method thereof.